Monitoring the Course of an Esterification Reaction by FTIR
Objective
Acetic acid and ethanol react to form ethyl acetate in the presence of a
mineral acid catalyst:
CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
With 1% H2SO4 as the catalyst the reaction will come to equilibrium in
about 20 minutes. You will monitor the change in concentration of each of
the four components over time using FTIR.
Instrumentation
KVB Analect Diamond-20 FTIR equipped with a DPR-210 ATR Deep
Immersion Probe.
Computer Software
Analect FX80 spectrometer control program.
FSP-6701 Multicomponent Analysis software.
Background
Beer’s Law states that the concentration of an absorbing species is
related to its absorbance by A = bc. However, this form of Beer’s Law is
valid only for a single species at a single wavelength. In a multicomponent
system, Beer’s Law is additive and the absorbance at a single frequency is
equal to the sum of absorbances for each component.
A Beer’s Law analysis of a multicomponent system generates a series
of equations that are best evaluated using matrix algebra. Several matrix
forms have been developed for multicomponent analysis. The most widely
used is the inverse least-squares method (aka the P-matrix). If you wish to
know the mathematics used in building a P-matrix, several resources are
available. The main advantage of the P-matrix is that impurities present in
the analysis mixture do not have to be quantified; however any impurities in
the analysis mixture must also be present in the calibration standards. The
quality of the calibration standards is the single most important factor for
producing meaningful results.
Procedure
Power up the interferometer and open FX80. At the bottom of the
screen you will see a series of command buttons. Within each command
button are various function buttons. Within each function button are
various parameter input fields. A detailed explanation of all command,
function and parameter options can be found in the FX80 User’s Guide.
Select the collect command button; the various function buttons are
displayed. Select the AQPARM button. The displayed parameter fields
contain information defining the Diamond-20’s physical configuration. This
information is stored on an EEPROM (electrically erasable programmable
read-only memory) chip in the instruments’ electronics control module. You
will not have to change any of these parameters.
Acquiring a Background
Close AQPARM and select Collect  AQBK. Select destination.
Type a file name for your background spectrum, choose C:\FTIR\ch_4212 as
the destination directory and press OK. Change the number of scans to 64.
You may also give your background spectrum a title and description. With
the dipper probe open to air, press OK to acquire a background spectrum.
After a short pause the scanning will start. When all the scans have been
collected, the window will close and the background spectrum will be
displayed. The background spectrum is now saved as a *.bkg file in the
C:\FTIR\ch_4212 directory. Close the window.
Acquiring Component Spectra
Immerse the dipper probe in pure deionized water. Select Collect 
AQSP. Type a file name for the transmission spectrum and choose
C:\FTIR\ch_4212 as the destination directory. Change the number of scans
to 64. Type in a title and description and press OK. After a short pause the
scanning will start. When all the scans have been collected, the window will
close and the water spectrum will be displayed. It is saved as a *.asf file in
the C:\FTIR\ch_4212 directory. Maximize the window. Select File I/O 
PRINT  OK to print the spectrum. Now select Analyze  PEAK 
calculate. A table will be generated with the wavenumber of each peak
maximum (x) and its’ associated transmission value (y). Select print
results to print the table. Repeat this procedure for the remaining three
components. Using the component data tables, identify a unique, strongly
absorbing peak frequency for each component (recall the relationship
between transmission and absorbance). The Mcomp program will use these
peak frequencies to build the P-matrix and analyze the reaction spectra.
Acquiring Calibration Spectra
Carefully prepare four calibration standards. Each standard must
contain all four components. The composition (proportions of components)
of each standard must be unique. The four standards should bracket the
expected concentration ranges. Prepare 200 ml of each. Acquire and save
the spectrum of each standard.
Building the P-Matrix
Close FX80 and exit windows. At the C:\FTIR\USER0 command
prompt type mcomp and select P-Matrix. Follow the instructions detailed
starting on page 43 of the FSP-6701 Multicomponent Analysis Software
User’s Guide. After building the matrix evaluate it with the Evaluate
analysis error function. A P-matrix built with carefully prepared
calibration standards will have component % errors of less than 2%. If any
of your component % errors are greater than 10%, re-build the matrix with
more carefully prepared standards and/or different peak frequencies. Be
sure to save the analysis file.
Running the Reaction
When you are satisfied with your P-matrix, exit mcomp and open
FX80 (type win at the C:\FTIR\USER0 prompt to load windows). Set up
the reaction apparatus as diagrammed below. Charge the reaction vessel
with 350 ml (70%) ethanol, 150 ml (30%) acetic acid and 5 ml cH2SO4.
Bring the mixture to reflux temperature (~75 C) and collect spectra every
two minutes for twenty minutes. Be sure to save the spectra and record the
time (minutes after reaching reflux) each was acquired.
Probe
Reflux Condenser
Thermometer
1L
3-neck flask
Heating Mantle
Stir Plate
Analyze the Reaction Spectra
Open Mcomp and follow the instructions on page 60 of the Mcomp
manual. Print the results of each reaction spectrum.
Report
In your report include the original prints of your P-matrix error
evaluation and reaction analysis data. Produce a plot of component
concentration vs. reaction time (all four on the same graph). How do your
results compare to the theoretical equilibrium concentrations?